Wireless ground link-based aircraft data communication system with roaming feature

ABSTRACT

A system and method for providing a retrievable record of the flight performance of aircraft includes a ground data link unit that obtains flight performance data representative of aircraft flight performance during flight of the aircraft. A spread spectrum transceiver, such as a frequency hopping or direct sequence spread spectrum transceiver, is coupled to a data store. A transmitter is operative to download the flight performance data that has been accumulated and stored by the data store during flight over the spread spectrum communication signal. A ground based spread spectrum transceiver receives the spread spectrum communication signal from the aircraft and demodulates the signal to obtain the flight performance data.

This application is a continuation of Ser. No. 09/474,894 filed on Jun.2, 1999, U.S. Pat. No. 6,154,637 which is a continuation of Ser. No.08/557,269, filed Nov. 14, 1995, now U.S. Pat. No. 6,047,165, issuedApr. 4, 2000, the disclosure of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates in general to communication systems, andis particularly directed to an aircraft data communication system havinga plurality of wireless ground links that link respectiveaircraft-resident subsystems, in each of which a copy of its flightperformance data is stored, with airport-located ground subsystems, eachground subsystem being coupled, in turn, by way of respective telcolinks to a remote flight operations control center, where flightperformance data from plural aircraft parked at different airports maybe analyzed and from which the uploading of in-flight data files may bedirected by airline systems personnel.

BACKGROUND OF THE INVENTION

Modern aircraft currently operated by the commercial airline industryemploy airborne data acquisition (ADA) equipment, such as a digitalflight data acquisition unit (DFDAU) as a non-limiting example, whichmonitor signals supplied from a variety of transducers distributedthroughout the aircraft, and provide digital data representative of theaircraft's flight performance based upon such transducer inputs. Asflight performance data is obtained by the acquisition equipment, it isstored in an attendant, physically robust, flight data recorder(commonly known as the aircraft's “black box”), so that in the unlikelyevent of an in-flight mishap, the flight data recorder can be removedand the stored flight performance data analyzed to determine the causeof the anomaly.

In a further effort to improve aircraft safety, rather than wait for anaccident to happen before analyzing flight recorder data, the FederalAviation Administration (FAA) has issued a draft advisory circularAC-120-XX, dated Sep. 20, 1995, entitled “Flight Operational QualityAssurance Program” (FOQA), which recommends that the airlines look atthe information provided by the digital flight data acquisition unit atregular intervals.

One suggested response to this recommendation is to equip each aircraftwith a redundant flight data recording unit having a removable datastorage medium, such as a floppy disc. Such an auxiliary digital datarecorder is intended to allow aircraft safety personnel to gain accessto the flight performance data by physically removing the auxiliaryunit's data disc, the contents of which can then be input to an aircraftperformance analysis data processing system for evaluation.

Although installing such a redundant flight data recording unit allowsairline personnel to retrieve a copy of the flight performance data forsubsequent evaluation, when considering the large volume of aircrafttraffic experienced by major commercial airports, the above proposedscheme is not only extremely time and manpower intensive, but is proneto substantial misidentification and aircraft/data association errors.

Other proposals, described in U.S. Pat. No. 5,359,446, are to use eithera direct line-of-sight infrared link or a fiber optic cable to couple anon-board aircraft computer system with a ground-based computer system.Obvious drawbacks to these systems are the fact that not only do theyemploy complex and expensive components, but require that the aircraftbe parked at the gate, so that the line-of-sight infrared transceiversor the fiber optic connection assemblies can be properly interlinked. Asa consequence, neither of these types of systems is effective for usewith commuter, cargo or military aircraft, which are customarily parkedon an apron, rather than at a mating jetway, where such an optical linkis to be provided.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above-described objectiveof periodically analyzing flight performance data, without having tophysically access a redundant unit on board the aircraft, issuccessfully addressed by means of a wireless ground data link, throughwhich flight performance data provided by airborne data acquisitionequipment is stored, compressed, encrypted and downloaded to anairport-resident ground subsystem, which forwards flight performancedata files from various aircraft to a flight operations control centerfor analysis. For purposes of providing a non-limiting example, in thedescription of the present invention, the data acquisition equipmentwill be understood to be a DFDAU.

For this purpose, an auxiliary data path is coupled from the DFDAU inparallel with the flight data recorder to a bidirectional radiofrequency (RF) carrier-based ground data link (GDL) unit, that isinstalled in the avionics compartment of the aircraft. The GDL unit isoperative to communicate with an airport-resident ground subsystem viathe RF communications ground link infrastructure.

In accordance with a preferred embodiment of the invention, thiswireless ground data link is implemented as a spread spectrum RF link,preferably having a carrier frequency lying in a reasonably wide (on theorder of 100 MHz) unlicensed 2.4-2.5 GHz S-band segment, which providesthe advantage of global acceptance. A benefit of spread spectrummodulation is its inherently low energy density waveform properties,which are the basis for its acceptance for unlicensed productcertification. Spread spectrum also provides the additional benefits ofresistance to jamming and immunity to multipath interference.

A principal function of the GDL unit is to store a compressed copy ofthe (ARINC 717) flight performance data generated by the DFDAU andsupplied to the aircraft's flight data recorder. The GDL unit is alsoconfigured to store and distribute auxiliary information uploaded to theaircraft from a wireless router (as directed by the remote operationscontrol center) in preparation for its next flight. The uploadedinformation may include audio, video and data, such as flight navigationinformation, and digitized video and audio files that may be employed aspart of an in-flight passenger service/entertainment package. The GDLunit may also be coupled to an auxiliary printer that is ported to theGDL unit in order to enable an immediate hard copy of flight datainformation (e.g. exceedences of parameter data) to be provided to thecrew immediately upon the conclusion of the flight.

Once an aircraft has landed and is within communication range of theground subsystem, the wireless router receives flight performance datavia the wireless ground data link from an aircraft's GDL unit. It alsosupplies information to the aircraft in preparation for its next flight.The wireless router receives flight files from the aircraft's GDL unitand forwards the files to an airport base station, which resides on theairport's local area network (LAN).

The airport base station forwards flight performance data files fromvarious aircraft by way of a separate communications path such as atelephone company (telco) land line to a remote flight operationscontrol center for analysis. The airport base station automaticallyforwards flight summary reports, and forwards raw flight data files,when requested by a GDL workstation.

The flight operations control center, which supports a variety ofairline operations including flight operations, flight safety,engineering and maintenance and passenger services, includes a systemcontroller segment and a plurality of FOQA workstations through whichflight performance system analysts evaluate the aircraft data files thathave been conveyed to the control center.

Depending upon its size and geographical topography, an airport mayinclude one or more wireless routers, that are installed within terminalbuildings serving associated pluralities of gates, to ensure completegate coverage. Redundant base stations may be utilized to assure highsystem availability in the event of a hardware failure. A largecommercial airport exhibits the communication environment of a smallcity; consequently, it can be expected that radio communications betweena respective wireless router and associated aircraft at gates will besubjected to multipath interference. In order to prevent the disruptionof wireless router-GDL communications as a result of such a multipathenvironment, the wireless ground data link between each aircraft and awireless router is equipped to execute either or both of a frequencymanagement and an antenna diversity scheme.

Antenna diversity, which may involve one or more diversity mechanisms,such as spatial or polarization diversity, ensures that an aircraft thathappens to be in a multi-path null of one antenna can still be incommunication with another antenna, thereby providing full systemcoverage regardless of blockage. Frequency management is accomplished bysubdividing a prescribed portion of the unlicensed radio frequencyspectrum used by the system for GDL-wireless router communications intoadjacent sub-band channels, and dynamically assigning such sub-bandchannels based upon the quality of the available channel links between arespective wireless router and a given aircraft. Such sub-channelassignments may involve downloading compressed and encrypted aircraftflight data over a first channel portion of the usable spectrum to thewireless router, and uploading information from a base station to theaircraft (e.g. video, audio and flight control data) from a wirelessrouter over a second channel portion of the useable spectrum to the GDLon board the aircraft.

In a preferred embodiment, a respective wireless router employs a sourcecoding system that achieves bandwidth reduction necessary to permiteither multiple audio channels to be multiplexed onto the wirelesstransmit carrier to the GDL unit, video to be transmitted over a groundsubsystem's wireless router-to-GDL unit ground link frequency channel,or data files to be compressed to maximize system throughput andcapacity during communications (uploads to or downloads from) theaircraft.

Cyclic Redundancy Check (CRC) coding is used for error detection only.When errors are detected at the wireless router, its transceiverrequests a retransmission from the GDL unit, in order to guarantee thatthe copy of the flight performance data file downloaded from the GDLunit and forwarded from a wireless router is effectively error free.

In the uplink direction from the ground subsystem to the aircraft, thebit error rate requirements for transmitting passenger entertainmentaudio and video files are less stringent, and a forward error correction(FEC) and error concealment mechanism is sufficient to achieve aplayback quality acceptable to the human audio/visual system. Also,since uploading an in-flight passenger audio/video file, such as a newsservice or entertainment program, may entail several tens of minutes(customarily carried out early in the morning prior to the beginning ofairport flight operations), there is usually no additional time for itsretransmission.

The wireless router transceiver includes a control processor whichensures robust system performance in the dynamically changing unlicensedspread spectrum interference environment of the ground data link bymaking decisions based on link signal quality, for the purpose ofsetting transmit power level, channel frequency assignment, and antennaselection. The ground subsystem processor also initiates aretransmission request to an aircraft's GDL unit upon detection of a biterror in a downlinked flight performance data packet.

Before requesting retransmission of a flight data packet, the wirelessrouter's transceiver measures the signal quality on the downlink channelportion of the ground data link. The transceiver in the wireless routerassesses the measured link quality, increases its transmit power levelas necessary, and requests a retransmission of the packet containing thebit error at a higher transmit power level. It then initiates aprescribed frequency management protocol, to determine if anotherchannel portion of the GDL link would be a better choice. If a higherquality channel is available, both transceivers switch over to the newfrequency. The flight performance data packet containing the bit erroris retransmitted until it is received error free at the wireless router.

Because the invention operates in an unlicensed portion of theelectromagnetic spectrum, it can be expected to encounter otherunlicensed communication products, such as employed by curbside baggagehandling and ticketing, rental car and hotel services, etc., therebymaking the communication environment unpredictable and dynamicallychanging. To solve this problem, the present invention employs afrequency management scheme, which initially determines the optimumoperating frequency and automatically changes to a better qualityfrequency channel when the currently established channel suffers animpairment.

The spread spectrum transceiver in each of an aircraft's GDL unit and anassociated airport wireless router includes a frequency agile spreadspectrum transmitter, a frequency agile spread spectrum receiver and afrequency synthesizer. In addition to being coupled to an associatedcontrol processor, the spread spectrum transmitter is coupled to anadaptive power control unit and an antenna diversity unit. Such a powerallocation mechanism makes more efficient use of available powersources, reduces interference, and makes more efficient use of theallocated frequency spectrum. The control processors at each end of thewireless ground link execute a communication start-up protocol, throughwhich they sequentially evaluate all of the available frequency channelsin the unlicensed 2.4-2.5 GHz S-band segment of interest and assess thelink quality of each of these channels.

Each wireless router transceiver sequentially and repeatedly sends out aprobe message directed to any of the GDL units that are within thecommunication range of gates served by that wireless router, on each ofall possible frequency channels into which the 2.4-2.5 GHz S-band spreadspectrum bandwidth has been divided. Each GDL unit within communicationrange of the wireless router returns a response message on eachfrequency channel, and indicates which frequency is preferred, basedupon the signal quality assessment and measured signal quality by itscommunication processor. The wireless router control processor evaluatesthe responses from each of the GDL units, selects the frequency ofchoice, and then notifies the GDL units within communication range ofits decision. This process is periodically repeated and is executedautomatically in the event of a retransmission request from a GDL unit.

As described earlier, in an environment such as a large commercialairport, a common cause of reduced signal quality is multipathinterference resulting from sudden attenuation in the direct pathbetween the transmitters and the receivers in the wireless router andaircraft, in conjunction with a delayed signal arriving at the receiverfrom a reflected path. This sudden attenuation in the direct pathbetween the aircraft and the wireless router can result in thedestructive summation of multiple paths at the antenna in use, resultingin a severe signal fading condition. The nature of multipath is suchthat switching to a second spatially separated or orthogonally polarizedantenna can result in a significant improvement in link performance.Since the wireless networking environment of an airport is one in whichobjects are likely to be moving between the wireless router and theaircraft, and one of the platforms (the aircraft) is mobile, antennadiversity can make the difference between reliable and unreliable systemperformance.

Pursuant to the invention, upon the occurrence of a prescribed reductionin link quality, an antenna diversity mechanism is employed. Such amechanism may involve the use of separate transceivers (each having arespective antenna), or an antenna diversity unit that switches betweena pair of spatially separated or orthogonally polarized antennas. Linkperformance is evaluated for each antenna in real time, on apacket-by-packet basis, to determine which antenna provides the bestreceive signal quality at the wireless router. Signal quality iscontinually measured at the receiver demodulator output and reported tothe control processor. Should there be a sudden degradation in linksignal quality, the wireless router control processor switches over tothe other antenna. If the degradation in signal quality cannot becorrected by invoking the antenna diversity mechanism, such as byswitching antennas, the wireless router has the option of increasing thetransmit power level at both ends of the link to compensate for thereduction in link quality and/or execute the frequency managementroutine to search for a better operating channel. In the wirelessrouter's broadcast mode, the same signal can be transmitted from bothantennas in order to assure reliable reception at all aircraft,regardless of changing multipath conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates the overall system architecture ofthe wireless ground link-based aircraft data communication systemaccording to the present invention;

FIG. 1A diagrammatically illustrates a non-limiting example of where,within the terminal topography of Atlanta's Hartsfield InternationalAirport, various subsystem portions of the system architecture of FIG. 1may be installed;

FIG. 1B diagrammatically illustrates a modification of FIG. 1A showingvarious subsystem portions of the system architecture of FIG. 1installed within the terminal topography of Atlanta's HartsfieldInternational Airport;

FIG. 1C lists identifications of the subsystem components of FIGS. 1, 1Aand 1B;

FIG. 2 diagrammatically illustrates a respective aircraft GDL segment ofthe system of FIG. 1;

FIG. 3 diagrammatically illustrates a GDL data storage andcommunications unit of a respective GDL segment of FIG. 2;

FIG. 4 diagrammatically illustrates the gate/terminal topography of theDallas/Fort Worth International Airport;

FIG. 5 diagrammatically illustrates a wireless router;

FIG. 6 diagrammatically illustrates the architecture of the wirelessrouter of FIG. 5 in greater detail;

FIG. 7 details the components of a spread spectrum transceiver; and

FIG. 8 diagrammatically illustrates a non-limiting example of afrequency channel subdivision of a spread spectrum transceiver of FIG.7.

DETAILED DESCRIPTION

Before describing in detail the wireless ground link-based aircraft datacommunication system in accordance with the present invention, it shouldbe observed that the present invention resides primarily in what iseffectively a prescribed arrangement of conventional avionics andcommunication circuits and associated digital signal processingcomponents and attendant supervisory control circuitry therefor, thatcontrols the operations of such circuits and components. Consequently,the configuration of such circuits and components and the manner inwhich they are interfaced with other communication system equipmenthave, for the most part, been illustrated in the drawings by readilyunderstandable block diagrams, which show only those specific detailsthat are pertinent to the present invention, so as not to obscure thedisclosure with details which will be readily apparent to those skilledin the art having the benefit of the description herein. Thus, the blockdiagram illustrations are primarily intended to show the majorcomponents of the system in a convenient functional grouping, wherebythe present invention may be more readily understood.

Referring now to FIG. 1, the overall system architecture of the wirelessground link-based aircraft data communication system according to thepresent invention is shown as being comprised of three interlinkedsubsystems: 1)—an aircraft-installed ground data link (GDL) subsystem100; 2)—an airport-resident ground subsystem 200; and 3)—a remote flightoperations control center 300. Associated with FIG. 1 are FIGS. 1A and1B, which diagrammatically illustrate non-limiting examples of where,within the terminal topography of Atlanta's Hartsfield InternationalAirport, various subsystem portions of the system architecture of FIG. 1may be installed. FIG. 1A shows overlapping antenna coverage frommultiple sites, while FIG. 1B shows full antenna coverage from a singletower. The subsystem portions are identified by the abbreviations listedin FIG. 1C, and referenced below.

The aircraft-installed ground data link (GDL) subsystem 100 is comprisedof a plurality of GDL airborne segments 101, each of which is installedin the controlled environment of the avionics compartment of arespectively different aircraft. Each GDL airborne segment 101 isoperative to communicate with a wireless router (WR) segment 201 of theairport-resident ground subsystem 200 through a wireless communicationslink 120.

The wireless router segment 201 routes the files it receives from theGDL airborne segment 101, either directly to the airport base station202 via the wired Ethernet LAN 207, or indirectly through local areanetworks 207 and airport-resident wireless bridge segments 203. Inaccordance with a preferred embodiment of the invention, the wirelesscommunication link 120 is a spread spectrum radio frequency (RF) linkhaving a carrier frequency lying in an unlicensed portion of theelectromagnetic spectrum, such as within the 2.4-2.5 GHz S-band.

As will be described, once installed in an aircraft, the data terminalequipment (DTE) 102 of a GDL segment 101 collects and stores flightperformance data generated on board the aircraft during flight. It alsostores and distributes information uploaded to the aircraft via a groundsubsystem's wireless router 201 (shown in detail in FIG. 5, to bedescribed) which is coupled thereto by way of a local area network 207from a base station segment 202 of a ground subsystem 200 in preparationfor the next flight or series of flights.

The uploaded information, which may include any of audio, video anddata, typically contains next flight information data, such as a set ofparameter-exceedence limits, and next flight navigation information,including, but not limited to, a navigation database associated with theflight plan of the aircraft, as well as digitized video and audio filesthat may be employed as part of a passenger service/entertainmentpackage.

The ground subsystem 200 includes a plurality of airport-resident GDLwireless router segments 201, one or more of which are distributedwithin the environments of the various airports served by the system. Arespective airport wireless router 201 is operative to receive andforward flight performance data that is wirelessly downlinked from anaircraft's GDL unit 101 and to supply information to the aircraft inpreparation for its next flight, once the aircraft has landed and is incommunication with the wireless router. Each ground subsystem wirelessrouter 201 forwards flight files from the aircraft's GDL unit andforwards the files to a server/archive computer terminal 204 of theaircraft base station 202, which resides on the local area network 207of the ground subsystem 200.

The airport base station 202 is coupled via a local communications path207, to which a remote gateway (RG) segment 206 is interfaced over acommunications path 230, to a central gateway (CG) segment 306 of aremote flight operations control center 300, where aircraft data filesfrom various aircraft are analyzed. As a non-limiting examplecommunications path 230 may comprise an ISDN telephone company (telco)land line, and the gateway segments may comprise standard LANinterfaces. However, it should be observed that other communicationmedia, such as a satellite links, for example, may be employed forground subsystem-to-control center communications without departing fromthe scope of the inventions

The flight operations control center 300 includes a system controller(SC) segment 301 and a plurality of GDL workstations (WS) 303, which areinterlinked to the systems controller 301 via a local area network 305,so as to allow flight performance systems analysts at control center 300to evaluate the aircraft data files conveyed to the flight operationscontrol center 300 from the airport base station segments 202 of theground subsystem 200.

The respective GDL workstations 303 may be allocated for differentpurposes, such as aircraft types (wide body, narrow body and commuteraircraft, for example). As described briefly above, the server/archiveterminal 204 in the base station segment 202 is operative toautomatically forward flight summary reports downloaded from an aircraftto the flight control center 300; it also forwards raw flight data fileswhen requested by a GDL workstation 303.

The system controller 301 has a server/archive terminal unit 304 thatpreferably includes database management software for providing forefficient transfer and analysis of data files, as it retrievesdownloaded files from a ground subsystem. As a non-limiting example,such database management software may delete existing files from a basestation segment's memory once the files have been retrieved.

In addition, at a respective ground subsystem 200, for a given aircraft,a batch file may be written into each directory relating to thataircraft's tail number, type and/or airline fleet, so that a GDL unit onboard the aircraft will be automatically commanded what to do, once aground data link has been established with a ground subsystem's wirelessrouter. The systems analyst at a respective GDL workstation 303 in theflight operations control center may initially request only a copy ofthe exceedence list portion of the flight parameter summary report.Should the report list one or more parameter exceedences, the systemanalyst may access the entire flight performance file relating to suchparameter exceedences.

Referring now to FIG. 2, a respective GDL segment 101 isdiagrammatically illustrated as comprising a GDL data storage andcommunications unit 111 (hereinafter referred to simply as a GDL unit,to be described with reference to FIG. 3) and an associated externalairframe (e.g. fuselage)-mounted antenna unit 113. In an alternativeembodiment, antenna unit 113 may house diversely configured components,such as spaced apart antenna dipole elements, antenna components.

The GDL unit 111 is preferably installed within the controlledenvironment of an aircrafts avionics compartment, to which communicationlinks from various aircraft flight parameter transducers, and cockpitinstruments and display components, shown within broken lines 12, arecoupled. When so installed, the GDL unit 111 is linked via an auxiliarydata path 14 to the aircraft's airborne data acquisition equipment 16(e.g. a DFDAU, in the present example). The GDL unit 111 synchronizeswith the flight parameter data stream from the DFDAU 16, and stores thecollected data in memory. It is also coupled via a data path 15 tosupply to one or more additional aircraft units, such as navigationalequipment and/or passenger entertainment stations, various data, audioand video files that have been uploaded from an airport ground subsystemwireless router 201.

The airborne data acquisition unit 16 is coupled to the aircraft'sdigital flight data recorder (DFDR) 18 by way of a standard flight datalink 19 through which collected flight data is coupled to the flightdata recorder in a conventional manner. In order to enable an immediatehard copy of prescribed flight data information (e.g. exceedences ofparameter data) to be printed out for review by the flight crewimmediately upon the conclusion of a flight, the cockpit-residentequipment may include an auxiliary printer 21 that is ported to GDL unit111.

As described briefly above, and as diagrammatically illustrated in FIG.3, GDL unit 111 is a bidirectional wireless (radio frequencycarrier-based) subsystem containing a processing unit 22 and associatedmemory 24 coupled to the DFDAU 16, via data path 14, which is parallelto or redundant with the data path to the flight data recorder 18.Processing unit 22 receives and compresses the same flight performancedata that is collected by the aircraft's digital flight data recorder,and stores the compressed data in associated memory 24. The compresseddata file includes a flight summary report generated by the processingunit 22, that includes a list of exceedences as defined by the parameterexceedence file.

To provide bidirectional RF communication capability with a wirelessrouter 201, GDL unit 111 includes a wireless (RF) transceiver 26, whichis coupled to the antenna unit 113. Preferably, memory 24 of the GDLunit 111 has sufficient archival storage capacity to retain accumulatedflight data files until the next landing, so that there is no loss offlight data due to airport terminal multipath or single point hardwarefailures, a requirement that all airports be equipped with a GDL system.

As will be described, on each of a plurality of sub-band channels of theunlicensed 2.4-2.5 GHz S-band segment of interest, a wireless router 201continuously broadcasts an interrogation beacon that containsinformation representative of the emitted power level restrictions ofthe airport. Using an adaptive power unit within its transceiver, theGDL unit 111 on board the aircraft responds to this beacon signal byadjusting its emitted power to a level that will not exceedcommunication limitations imposed by the jurisdiction governing theairport. The wireless (RF) transceiver 26 then accesses the compressedflight performance data file stored in memory 24, encrypts the data andtransmits the file via a selected sub-channel of the wireless groundcommunication link 120 to wireless router 201. The sub-channel selectedis based upon a signal quality monitoring mechanism, as will bedescribed. The recipient wireless router 201 forwards the data file tothe base station segment for storage; further, the flight summary fileis automatically transmitted over the communications path 230 to theremote flight operations control center 300 for analysis.

As noted above, each airport-resident subsystem 200 of the presentinvention comprises one or a plurality of ground subsystem wirelessrouters 201. The number of wireless routers 201 installed at any givenairport and the location of each ground subsystem within thegeographical confines of the airport is preferably tailored inaccordance with a number of factors, such as the topography of theairport, including the location of a tower relative to a terminal'sgates, and a desired location of wireless router that facilitates accessto communication path 230 to the remote flight operations control center300.

Typically, but not necessarily, a wireless router 201 may be physicallyinstalled at a (roof) location of an airport terminal building serving aplurality of gates, such as location 211 in the familiar‘multi-horseshoe’ topography of the Dallas/Fort Worth InternationalAirport, diagrammatically illustrated in FIG. 4, as a non-limitingexample. Where an airport contains multiple terminals or has a largenumber of gates distributed over a substantial airport area (as does theDallas/Fort Worth International Airport), the airport may be equippedwith one or more additional wireless router locations, shown at 212 inFIG. 4, in order to ensure complete gate coverage.

The locations of wireless router locations 211 and 212 are such that,regardless of its location, each aircraft will be assured of having awireless ground data link with a wireless router of the groundsubsystem. In the exemplary environment of the Dallas/Fort WorthInternational Airport of FIG. 4, the spacing between wireless routerlocations 211 and 212 is such as to provide overlapping ground linkcommunication coverage, as indicated by overlapping circles 214 and 215,whose respective radii encompass the entirety of their associatedmulti-gate areas 216 and 217. (Similar overlapping circle coverage isdiagrammatically shown in FIG. 1A for wireless routers located atconcourses A and of the Atlanta airport, as another non-limitingexample.)

Because a large airport, such as each of the Atlanta and Dallas/FortWorth International Airports, has multiple terminal and maintenancebuildings, and a sizeable number of ground service vehicles andpersonnel, serving multiple, various sized aircraft, from private,single engine aircraft to jumbo jets, the airport effectively exhibitsthe communication environment of a small city. As a result, it can beexpected that radio communications between a respective wireless routerand its associated gates will be subjected to multipath interference.

In order prevent the disruption of wireless router-GDL unitcommunications in such a multipath environment, the wirelesscommunication links that are established between the aircraft and theground subsystem wireless routers preferably employ a frequencymanagement and a diversity antenna scheme that optimizes the choice offrequency channel within the available unlicensed 2.4-2.5 GHz S-bandemployed in accordance with the invention.

As noted earlier, antenna diversity may involve the use of separatetransceivers (each having a respective antenna), or an antenna diversityunit that switches between a pair of spatially separated or orthogonallypolarized antennas, as non-limiting examples, so as to ensure that anaircraft that happens to be located in a multi-path null of one antennacan still be in communication with another antenna, thereby providingfull system coverage regardless of blockage or multi-path nulls.

For this purpose, as diagrammatically shown in FIG. 5, a respectivewireless router 201 may include an RF transceiver 221 having a pair ofassociated first and second antennas 222 and 223, which may be mountedon the roof of a terminal building, as noted above, so as to bephysically spaced apart from one another (either vertically,horizontally, or both) by a prescribed separation distance that issufficient to provide antenna spatial diversity. As a non-limitingexample, for an RF carrier frequency in the unlicensed 2.4-2.5 GHzS-band, spacing antennas 222 and 223 apart from one another by adistance on the order of ten feet has been found to satisfactorilyobviate multipath interference. As will be described in greater detailbelow with reference to FIG. 6, transceiver 221 has an associatedcommunications processor 225 which is coupled via communications path230 to the remote flight control center 300.

The redundant coverage provided by the diversity antenna mechanismensures that should an aircraft be located in a multi-path null of oneantenna, that particular aircraft can still be seen by the otherantenna, thereby providing full wireless router coverage regardless ofblockage. In addition, where an additional wireless router is provided,system reliability can be enhanced to provide a high probability ofsuccessful communications, should a single point hardware failure occur.This added redundancy prevents a single wireless router failure fromsevering the GDL airport system coverage, and delaying access to flightfiles. As pointed out above, in the unlikely event of a system failureat one GDL-equipped airport, the memory 24 of a respective GDL unit 111has sufficient archival storage capacity to retain accumulated flightdata files until the next landing, so that there is no loss of flightdata due to airport terminal multipath or single point hardwarefailures.

The frequency management scheme employed by each of the wireless routerand GDL unit transceivers involves subdividing the unlicensed radiofrequency S-band spectral segment (2.4-2.5 GHz) used by the system forinter GDL-wireless router communications into adjacent sub-bandchannels, and dynamically assigning such sub-band channels, based uponthe quality of the available channel links between a respective wirelessrouter and a given aircraft. Such sub-channel assignments may involvedownloading compressed and encrypted aircraft flight data over a firstchannel portion of the usable spectrum to the wireless router, anduploading information to the aircraft (e.g. video, audio and flightcontrol data) from a wireless router 201 over a second channel portionof the useable spectrum to the GDL 111 on board the aircraft.

Pursuant to a preferred embodiment of the present invention, eachwireless router 201 employs a source coding system that achievesbandwidth reduction necessary to permit either multiple audio channelsto be multiplexed onto the wireless transmit carrier to an aircraft'sGDL unit 111, video to be transmitted over the wireless router-GDL unitground link frequency channel, or data files to be compressed in orderto maximize system throughput and capacity during upload to theaircraft. The primary advantage of source coding is data compression,which permits any audio, video, or data to be uploaded to the aircraftto be compressed and multiplexed onto a single RF carrier. Employingsource coding also eliminates the need for multiple, simultaneouscarriers, which increases channel assignment options, and translatesdirectly to improved link performance.

As pointed out earlier, the unlicensed frequency spectrum is becomingincreasingly crowded, so that expanding the number of channel assignmentoptions can mean the difference between being able to operate or not.Fewer transmitters also means lower power consumption, decreasedcomplexity, and improved reliability. Adjacent channel interferenceconcerns resulting from the close proximity of multiple frequencydivision multiplex transmitters is not an issue with a single carriersystem. As a non-limiting example, Motion Picture Expert Group (MPEG)coding may be employed for audio and video signals, while othersimilarly conventional compression algorithms (such as PKZiP) may beused for generic data file compression.

In order to provide a reliable bidirectional RF communication linkbetween the aircraft and the wireless router, namely one which is ableto withstand the effects of channel impairments such as noise, jamming,or fading, the wireless ground data link communication mechanism of thepresent invention employs an error detection and retransmission errorcorrection scheme to assure error free communications for downloadingflight performance data from the aircraft to a ground subsystem wirelessrouter. While exchanging flight-critical data files in theaircraft-to-wireless router direction, cyclical redundancy check (CRC)coding is used for error detection only. When errors in the downloadedflight data are detected at the wireless router 201, the wirelessrouter's transceiver requests a retransmission from the aircraft GDLunit. This fulfills the critical requirement that the copy of the flightdata file downloaded from the GDL unit and forwarded from the wirelessrouter must be effectively error free.

In the uplink direction from the wireless router 201 to the aircraft, onthe other hand, the bit error rate requirements for transmitting nonflight-critical data, such as passenger entertainment audio and videofiles, are less stringent, and a forward error correction (FEC)mechanism is sufficient to achieve a playback quality onboard theaircraft, that is acceptable to the human audio/visual system. Where thedata transmitted to the aircraft is flight critical, the error detectionand retransmission scheme as described above for the downlink directionis employed.

Moreover, because uploading an in-flight passenger audio/video file,such as a news service or entertainment program, may entail several tensof minutes (customarily carried out early in the morning prior to thebeginning of airport flight operations), there is usually no time forretransmission of such a large database. Typically, during this‘pre-ops’ time interval, with no arriving flights being handled, theentire bandwidth availability may be used for broadcasting one or morevideo news and entertainment files to multiple aircraft at the same time(using industry standard broadband coding such as MPEG, referencedabove).

The manner in which the above described error detection andretransmission error correction scheme may be implemented in arespective wireless router is diagrammatically illustrated in FIG. 6,which details the architecture of wireless router transceiver componentsand associated interfaces to other system segment components. The systemcontroller wireless router transceiver includes a multiplexer unit 241,containing system time synchronization circuitry and which is operativeto selectively interface one of first and second source coding units 243and 245 and a channel coding unit 247. The source coding units 243 and245 are coupled to respective external data interfaces, while codingunit 247 is interfaced with a wireless router control processor 225,which serves as a baseband interface between channel coding unit 247 anda spread spectrum transceiver 251 (to be described in detail below withreference to FIG. 7).

As described briefly above, and as will be detailed below, wirelessrouter control processor 225 is operative to ensure robust systemperformance in the unpredictable and dynamically changing unlicensedspread spectrum interference environment of the wireless ground datalink 120, by making decisions based on link signal quality, for settingtransmit power level, channel frequency assignment, and antennaselection. It also initiates a retransmission request to the GDL unit111 in the event of a bit error in a received (downloaded) flightperformance data packet.

More particularly, when a cyclic redundancy check (CRC) error in thedata stream received by the wireless router is detected by channelcoding unit 247, control processor 225 initiates a retransmissionrequest on the return channel portion of the wireless link 120 back tothe transceiver 26 within the aircraft's GDL unit 111. Before requestingretransmission of a flight data packet, the control processor 225measures the signal quality on the downlink channel portion of the link120. The wireless router 201 assesses measured link quality, increasesits transmit power level as necessary, and requests a retransmission ofthe flight performance data packet containing the bit error at a highertransmit power level. It then initiates a prescribed frequencymanagement protocol, to be described below with reference to FIG. 8, inorder to determine if another channel portion of the GDL link would be abetter choice. If a better (higher quality) channel is available, boththe GDL transceivers switch over to the new frequency channel (withinthe unlicensed 2.4-2.5 GHz S-band of interest). The packet containingthe bit error is retransmitted until it is detected by wireless routercontrol processor 225 as being error-free.

As noted previously, since the wireless ground data link system of thepresent invention operates in an unlicensed portion of the EM frequencyspectrum, it can be expected that it will encounter other unlicensedproducts, which are also permitted to roam without imposed geographic(site-licensing) constraints. As a consequence, the operatingenvironment is unpredictable and dynamically changing. The level ofactivity within this unlicensed portion of the EM frequency spectrum canbe expected to increase as more and more airport-related services, suchas curbside baggage handling and ticketing, rental car and hotelservices, etc., use compact (hand-held or headset-configured) unlicensedwireless communication devices.

This mutual interference effect is similar to that encountered in the HFfrequency band, where ionospheric radio links are subject to a number oftransmission quality degradation characteristics, such as multipath,Doppler, fading and temporary loss of signal. The unpredictability ofthis environment originates from the relatively long wavelength of thecarrier frequency and the fact that an HF radio wave bounces off theatmosphere, enabling it to propagate tremendous distances beyond thehorizon. As a result, interference from transmitters that aregeographically separated by great distances can pose problems. Since theionosphere varies in height and ionization with time of day, season, andthe solar cycle, the constantly changing interference characteristics ofthe HF environment are difficult to predict. It will be appreciated,therefore, that there are a number of similarities between operating inthe HF band and operating in an unlicensed frequency band.

To solve this problem, the present invention employs a frequencymanagement scheme, which initially determines the optimum operatingfrequency for the GDL link, and automatically changes to a betterquality frequency channel when the currently established channel suffersan impairment. Such a frequency management scheme effectivelycorresponds to that employed in the U.S. Pat. to D. McRae et al, No.4,872,182, entitled, “Frequency Management System for Use inMultistation H.F. Communication Network,” assigned to the assignee ofthe present application and the disclosure of which is incorporatedherein.

For this purpose, the spread spectrum transceiver of the presentinvention, which may be employed in the transceiver 251 of the wirelessrouter of FIG. 6 and also in the transceiver 26 of an aircraft's GDLunit 111, is shown in more detail in FIG. 7 as comprising a frequencyagile spread spectrum transmitter 253, a frequency agile spread spectrumreceiver 255 and a frequency synthesizer 257. In addition to beingcoupled to an associated control processor, the spread spectrumtransceiver 251 is coupled to RF components, including an adaptive powercontrol unit 252 and an antenna diversity unit 254, as will bedescribed. As a non-limiting example, such spread spectrum transceivercomponents may be implemented using a direct sequence spread spectrumwireless transceiver chipset and associated signal processingcomponents, of the type as described the Harris Semiconductorinformation bulletins entitled: “PRISM (trademark Harris Corp.) 2.4 GHzChip Set,” Apr. 1995, “HFA3624 2.4 GHz RF to IF Converter,” Feb. 14,1995, “HFA3724 400 MHz Quadrature IF Modulator/Demodulator,” February,1995, “HSP3824 Direct Sequence Spread Spectrum Baseband Processor,” Mar.1995, and “HFA3924 2.4 GHz Power Amplifier,” Feb. 13, 1995.

The respective control processors at each end of the wireless grounddata link (control processor 225 in the wireless router and thecommunications processing unit 22 in the GDL unit 111) employ acommunication control mechanism that executes a start-up protocol,whereby all available frequency channels are examined to determine thelink quality of each channel. For this purpose, the wireless routertransceiver broadcasts out a probe message to each of the GDL units thatare within communication range of gates served by that wireless router,in sequence, on each of all possible frequency channels into which the2.4-2.5 GHz spread spectrum S-bandwidth has been divided, as showndiagrammatically in FIG. 8. These probe messages are repeated apredetermined number of times.

Each sequentially interrogated GDL unit 111 then returns a responsemessage on all the frequency channels, indicating which frequency ispreferred, based upon the signal quality assessment and measured signalquality by its communication processor 22. The wireless router controlprocessor 225 evaluates the responses from each of the GDL units 111,selects the frequency of choice, and then notifies each GDL unit 111within communication range of its decision. This process is periodicallyrepeated and is executed automatically in the event of a retransmissionrequest from a GDL unit 111, as a result of a detected bit error, asdescribed above.

As those skilled in the art are aware, a spread spectrum signal is oneoccupying a bandwidth much greater than the minimum bandwidth necessaryto send information contained in the spread signal. Spreading of atransmitted signal across the bandwidth of interest is accomplished byuse of a spreading code, or pseudo-random noise (PN) sequence, which isindependent of the information being transmitted. At the receiver,despreading of the spread signal is accomplished by correlating thereceived signal with a matched replica of the spreading code used in thetransmitter. Although implementation complexity and associated productcost have constituted practical impediments to the use of spreadspectrum communications outside of niche military markets, recentadvances in integrated circuit manufacturing techniques have now made itpossible to provide reasonably priced spread spectrum communicationcircuits so that they may be employed in a variety of otherapplications.

In accordance with the present invention the spread spectrum transmitterand receiver components have two particularly useful characteristics.The first is their operation in the 2.4-2.5 GHz unlicensed S-band, whichprovides both the user and the manufacturer the advantages of globalunlicensed operation. Other alternatives restrict usage geographicallyor require the user to obtain a license in order to operate the system.In the United States, FCC compliance is governed by Part 15.247.

The second is the use of direct sequence spread spectrum (DSSS), asopposed to the use of frequency hopped or narrowband communications. Theinherent low energy density waveform properties of DSSS are the basisfor its acceptance for unlicensed product certification. DSSS alsoprovides the additional benefits of resistance to jamming and immunityto the multipath problem discussed above as a function of the amount ofspreading employed. Moreover, the number of orthogonal signal dimensionsof DSSS is larger than narrowband techniques, so that a sophisticatedreceiver is readily able to recognize and recover the intended signalfrom a host of potential interferers, thereby reducing their effect.

In the current wireless marketplace, where RF spectrum allocations havebecome a precious commodity, the prospects of unintentional jamming growincreasingly greater. Spread spectrum is a robust combatant to thegrowing threat of RF spectrum proliferation. Pursuant to the presentinvention, the DSSS transceivers employed in each of the GDL unit 111 onboard the aircraft and in the airport's ground subsystem wireless router201 are frequency agile, so that they can be tuned to any of a pluralityof frequency channels approved for unlicensed operation in a givencountry. DSSS also provides the attractive performance benefits ofimmunity against jamming from interferers and immunity againstself-jamming from multipath, as described earlier.

In order to provide orthogonal signal isolation from IEEE 802.11 users,it is preferred to employ a different PN code than the standard, butstill complying with strict regulatory guidelines required for typelicensing, such as FCC 15.247, referenced above. In addition, asdiagrammatically illustrated in the frequency channel subdivisiondiagram of FIG. 8, the DSSS transceiver of FIG. 7 may employ differenttransmit frequencies and a different channel spacing to minimizeco-channel interference. This mechanism is akin to that employed incellular telephone networks which make use of a return channel from acellular base station to allow a customer's handset to reduce itstransmit power to the minimum level required to maintain reliablecommunications. Such a power allocation mechanism prolongs battery life,reduces interference, and makes more efficient use of the allocatedfrequency spectrum.

In the transceiver architecture of FIG. 6 employed in the GDL system ofthe present invention, the signal quality (e.g., bit error rate) ismeasured by wireless router control processor 225 to sense channelimpairments. As described earlier, in an environment such as a largecommercial airport, a common cause of reduced signal quality ismultipath interference resulting from sudden attenuation in the directpath between the transmitter and the receivers in the wireless routerand aircraft, in conjunction with a delayed signal arriving at thereceiver from a reflected path. This sudden attenuation in the directpath between the aircraft and the wireless router can result in thedestructive summation of reflected paths at the antenna in use,resulting in a severe signal fading condition. The nature of multipathis such that switching to a second spatially separated or orthogonallypolarized antenna can result in a significant improvement in linkperformance. Since the wireless networking environment of an airport isone in which objects are likely to be moving between the wireless routerand the aircraft, and one of the platforms is mobile, the use of anantenna diversity unit can make the difference between reliable andunreliable system performance.

In the event of a prescribed reduction in link quality, antennadiversity unit 254 is operative under processor control to switchbetween a pair of spatially separated or orthogonally polarized antennas258 and 260. Link performance is evaluated for each antenna in realtime, on a packet-by-packet basis, to determine which antenna providesthe best receive signal quality at a ground subsystem's wireless router.Signal quality is continually measured at the receiver demodulatoroutput and reported to the control processor. In the event of a suddendegradation in link signal quality, the wireless router controlprocessor switches over to the other antenna. If the degradation insignal quality cannot be corrected by switching antennas, the wirelessrouter has the option of increasing the transmit power level at bothends of the link to compensate for the reduction in link quality and/orinitiate the frequency management protocol to search for a betteroperating channel. In the broadcast mode, the same signal can betransmitted from both antennas in order to assure reliable reception atall aircraft GDL units, regardless of changing multipath conditions.

If the transceiver is unable to produce a satisfactory improvement inlink quality by switching antennas in the manner described above, thenby way of the return channel, the control processor in the receivernotifies the transmitter of the condition and the measure of linkquality. The transmitter then assesses the magnitude of the channelimpairment as a result of examining the measured signal quality reportedback from the receiver and instructs the adaptive power control unit 252to increase its transmit power to compensate for the impairment, ifappropriate. If the impairment is so severe that the transmitter cannotcompensate for the impairment by increasing its transmit power level, itinitiates frequency management protocol to find a clear channel.

In the transceiver architecture of FIG. 6, the spread spectrum receiverunit 251 (shown in detail in the control processor 225. Signal qualitymeasurements are carried simultaneously with symbol timing measurementsand are declared when an acceptable signal is to be processed. Thesignal quality measured is a function of the average magnitude of the PNcorrelation peaks detected and of the time averaged phase error. Thetransceiver also performs a clear channel assessment, by monitoring theenvironment to determine when it is feasible to transmit. The wirelessrouter receiver makes real time antenna diversity decisions to choosethe best antenna to receive from on an aircraft by aircraft basis. Oncea decision is made, the same antenna is used for wireless routertransmissions back to the GDL unit in the aircraft, except in thebroadcast mode, where both antennas 258 and 260 are used simultaneously.

As will be appreciated from the foregoing description, the objective ofsatisfying the FAA's current airline Flight Operations Quality Assuranceprogram, which recommends that airlines routinely analyze aircraft data,is successfully addressed in accordance with the present invention bymeans of a frequency-agile wireless ground data link, that uses areasonably wide unlicensed portion of the EM spectrum, does not requirephysically accessing the aircraft, and supplies the same aircraft dataprovided by the airborne data acquisition unit in a compressed andencrypted format, that is automatically downloaded to anairport-resident base station segment, when the aircraft lands. Whenpolled by a remote flight operations control center, the base stationsegment then forwards aircraft data files from various aircraft over acommunication path such as a telco land line to the flight operationscontrol center for analysis.

While we have shown and described an embodiment in accordance with thepresent invention, it is to be understood that the same is not limitedthereto but is susceptible to numerous changes and modifications asknown to a person skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed is:
 1. A system for providing a retrievable record ofthe flight performance of an aircraft comprising: a ground data linkunit that obtains flight performance data representative of aircraftflight performance during flight of the aircraft, said ground data linkunit comprising: a) an archival data store operative to accumulate andstore flight performance data during flight of the aircraft, and b) afrequency hopping spread spectrum transceiver coupled to said archivaldata store, and comprising a transmitter that is operative to downloadthe flight performance data that has been accumulated and stored by saidarchival data store during flight over a frequency hopping spreadspectrum communication signal; a ground based spread spectrumtransceiver comprising a receiver that receives the frequency hoppingspread spectrum communication signal from the aircraft and demodulatesthe signal to obtain the flight performance data; a ground basedarchival data store coupled to said ground based spread spectrumtransceiver that receives and stores said flight performance data; and aground based processor coupled to said archival data store forretrieving flight performance data from the ground based archival datastore for further processing.
 2. A system according to claim 1, whereinsaid wideband spread spectrum transceiver is operative to download theflight performance data after the aircraft completes its flight andloads at an airport.
 3. A system according to claim 1, wherein theground based wideband spread spectrum transceiver further comprises awireless router that couples said wideband spread spectrum transceiverto said ground based archival data store.
 4. A system for providing aretrievable record of the flight performance of an aircraft comprising:a ground data link unit that obtains flight performance datarepresentative of aircraft flight performance during flight of theaircraft, said ground data link unit comprising: a) an archival datastore operative to accumulate and store flight performance data duringflight of the aircraft, and b) a direct sequence spread spectrumtransceiver coupled to said archival data store, and comprising atransmitter that is operative to download the flight performance datathat has been accumulated and stored by said archival data store duringflight over a direct sequence spread spectrum communication signal; aground based spread spectrum transceiver comprising a receiver thatreceives the direct sequence spread spectrum communication signal fromthe aircraft and demodulates the signal to obtain the flight performancedata; a ground based archival data store coupled to said ground basedspread spectrum transceiver that receives and stores said flightperformance data; and a ground based processor coupled to said archivaldata store for retrieving flight performance data from the ground basedarchival data store for further processing.
 5. A system according toclaim 4, wherein said wideband spread spectrum transceiver is operativeto download the flight performance data after the aircraft completes itsflight and lands at an airport.
 6. A system according to claim 4,wherein the ground based wideband spread spectrum transceiver furthercomprises a wireless router that couples said wideband spread spectrumtransceiver to said ground based archival data store.
 7. A system forproviding a retrievable record of the flight performance of an aircraftcomprising: a ground data link unit that obtains flight performance datarepresentative of aircraft flight performance during flight of theaircraft, said ground data link unit comprising: a) a data storeoperative to accumulate and store flight performance data during flightof the aircraft, and b) a frequency hopping spread spectrum transceivercoupled to said data store, and comprising a transmitter that isoperative to download the flight performance data that has beenaccumulated and stored by said archival data store during flight over afrequency hopping spread spectrum communication signal; and a groundbased spread spectrum transceiver comprising a receiver that receivesthe frequency hopping spread spectrum communication signal from theaircraft and demodulates the signal to obtain the flight performancedata.
 8. The system according to claim 7, and further comprising aground based data store coupled to said ground based spread spectrumtransceiver that receives and stores said flight performance data; and aground based processor coupled to said data store for retrieving flightperformance data from the ground based archival data store for furtherprocessing.
 9. A system according to claim 7, wherein the frequencyhopping spread spectrum transceiver is operative after the aircraftcompletes its flight to download the flight performance data.
 10. Asystem for providing a retrievable record of the flight performance ofan aircraft comprising: a ground data link unit that obtains flightperformance data representative of aircraft flight performance duringflight of the aircraft, said ground data link unit comprising: a) a datastore operative to accumulate and store flight performance data duringflight of the aircraft, and b) a direct sequence spread spectrumtransceiver coupled to said data store, and comprising a transmitterthat is operative to download the flight performance data that has beenaccumulated and stored by said data store during flight over a directsequence spread spectrum communication signal; and a ground based spreadspectrum transceiver comprising a receiver that receives the directsequence spread spectrum communication signal from the aircraft anddemodulates the signal to obtain the flight performance data.
 11. Thesystem according to claim 10, and further comprising a ground based datastore coupled to said ground based spread spectrum transceiver thatreceives and stores said flight performance data; and a ground basedprocessor coupled to said data store for retrieving flight performancedata from the ground based data store for further processing.
 12. Asystem according to claim 10, wherein the frequency hopping spreadspectrum transceiver is operative after the aircraft completes itsflight to download the flight performance data.
 13. A method ofproviding a retrievable record of the flight performance of an aircraftcomprising the steps of: acquiring flight performance data of anaircraft during flight of the aircraft; accumulating and storing withinan archival memory of a ground data link unit the flight performancedata during flight of the aircraft; downloading the flight performancedata that has been accumulated and stored within the archival data storeduring the flight over a frequency hopping spread spectrum communicationsignal to a ground based spread spectrum receiver; demodulating thereceived frequency hopping spread spectrum signal to obtain the flightperformance data; storing the demodulated flight performance data withina ground based archival data storage; and retrieving the flightperformance data via a ground based processor for further processing.14. A method of providing a retrievable record of the flight performanceof an aircraft comprising the steps of: acquiring flight performancedata of an aircraft during flight of the aircraft; accumulating andstoring within an archival memory of a ground data link unit the flightperformance data during flight of the aircraft; downloading the flightperformance data that has been accumulated and stored within thearchival data store during the flight over a direct sequence spreadspectrum communication signal to a ground based spread spectrumreceiver; demodulating the received spread spectrum signal to obtain theflight performance data; storing the demodulated flight performance datawithin a ground based archival data storage; and retrieving the flightperformance data via a ground based processor for further processing.15. A method of providing a retrievable record of the flight performanceof an aircraft comprising the steps of: acquiring flight performancedata of an aircraft during flight of the aircraft; accumulating andstoring within a memory of a ground data link unit the flightperformance data during flight of the aircraft; downloading the flightperformance data that has been accumulated and stored within thearchival data store during the flight over a frequency hopping spreadspectrum communication signal to a ground based spread spectrumreceiver; and demodulating the received spread spectrum signal to obtainthe flight performance data.
 16. A method of providing a retrievablerecord of the flight performance of an aircraft comprising the steps of:acquiring flight performance data of an aircraft during flight of theaircraft; accumulating and storing within a memory of a ground data linkunit the flight performance data during flight of the aircraft;downloading the flight performance data that has been accumulated andstored within the data store during the flight over a direct sequencespread spectrum communication signal to a ground based spread spectrumreceiver; and demodulating the received spread spectrum signal to obtainthe flight performance data.